1. Field of the Invention
The present invention relates to a liquid crystal display element.
2. Description of the Related Art
Conventionally, a TN (twisted namatic) liquid crystal display element which employs nematic liquid crystal or an STN (super twisted nematic) liquid crystal display element is practically used as a liquid crystal display element. Further, a liquid crystal display element which employs ferroelectric liquid crystal is conventionally proposed. However, in order to manufacture an element using any of the above-mentioned types of liquid crystals, it is necessary to carry out an alignment process and the like, and therefore a large-size element cannot be easily manufactured.
In the meantime, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 58-501632 or U.S. Pat. No. 4,435,047, a method of forming a liquid crystal layer by dispersing a capsulated liquid crystal droplet in polyvinyl alcohol, is known as a technique for manufacturing a large-size and low-cost liquid crystal display element without carrying out such an alignment process. With regard to elements obtained by the above method, in those elements whose liquid crystal molecules exhibit a positive dielectric anisotropy in a thin film, the liquid crystal molecules are arranged in the direction of an electric field when such an electrical field is applied. In this case, if the refractive index n.sub.o of the liquid crystal and the refractive index n.sub.p of the polymer are equal to each other, the liquid crystal layer becomes transparent. When the electrical field is removed, the liquid crystal molecules are set back in a random orientation, and a difference is created between the refractive indexes n.sub.o and n.sub.p of the liquid crystal droplet. Therefore, liquid crystal droplets scatter light as being reflected on their interfaces and the liquid crystal layer is made opaque in white.
Other than those described above, some other techniques for dispersing liquid crystals in polymer are known. For example, Jpn. Pat. Appln. KOKAI Publication No. 61-502128 discloses the dispersion of liquid crystals in epoxy resin, and Jpn. Pat. Appln. KOKAI Publication No. 2-83534 discloses the dispersion of liquid crystals in urethaneacrylate resin. Further, Jpn. Pat. Appln. KOKAI Publication No. 63-98631 discloses a liquid crystal display element having a parallel beam supplying section and a selective light transmitting section in its capsule liquid crystal section, as means for improving the contrast of the element.
These so-called polymer dispersion type liquid crystal entails the following advantages. That is, (1) No polarizing plate is required and therefore a bright image can be formed. (2) A wide viewing angle can be obtained. (3) Since the alignment process for the liquid crystal is not required, the creation of dust, the generation of static electricity and the like, which may result in the rubbing step, can be avoided. (4) As compared to the conventional liquid crystal display element in which liquid crystal is packed between substrates and sealed, the polymer dispersiontype liquid crystal element has a simple structure, and it can be easily increased in size.
However, at the same time, the polymer dispersion type liquid crystal display element entails the following disadvantages.
First, the disadvantages with regard to the manufacture thereof will be listed. A polymer dispersion liquid crystal display element is manufactured in the following fashion. That is, a thermosetting or photo-setting liquid resin in which liquid crystal substances are dispersed or dissolved, is poured inside a cell formed of two substrates having electrodes, and then the liquid resin is hardened by irradiation of heat or light, so as to obtain a solid resin in which liquid crystal droplets are dispersed.
Therefore, with this method, (1) due to a slight difference in the conditions of the irradiation of heat or light, the size or shape of the liquid crystal droplets is changed, and the operating voltage or the contrast of the product display element is rendered different from one case to another. Further, (2) since the resin is hardened, it is impossible to recover defects once the manufacture of the element is completed, whereas in the case of a TN or STN liquid crystal display element, it is still possible to recover alignment errors or the like by annealing after the element has been manufactured.
Next, the disadvantages in terms of electroptics will now be discussed. (1) When a voltage is applied to the liquid crystal display element, a hysteresis occurs. As a result, with the polymer dispersion liquid crystal display element, it is impossible to display a half-tone.
Recently, there has been a report on a lightscattering liquid crystal element which entails the advantages of the polymer dispersion liquid crystal and removes the above-described disadvantages. More specifically, Jpn. Pat. Appln. KOKAI Publication No. 5-216015 discloses a liquid crystal display element having a mixture material in a gel state, as a structural element, which contains a compound having a perfluoroalkyl group, and a liquid crystal substance.
This display element entails the following advantages. (1) It is possible to display without hysteresis. This is because a model of stepwise deformation of the liquid crystal within a liquid crystal droplet, which is the main cause for the hysteresis, is not established. (2) A contrast as high as that of a TN liquid crystal cell, that is, 1:50 to 1:80, can be realized. More specifically, as compared to an ordinary polymer dispersion liquid crystal, the scattering of light is increased when a voltage is not applied. Therefore, the light shading property is improved, and the contrast is increased. (3) V.sub.th is 2 to 4 V, and the element can be driven at a voltage as low as that of a TN liquid crystal cell.
However, even a liquid crystal display element which employs a compound containing a perfluoroalkyl group, entails the following disadvantages. The compound containing a perfluoroalkyl group has a low molecular weight, and therefore this liquid crystal display element has, for example, a low film forming ability and a low long-term stability of the phase separation state, as compared to the liquid crystal display element which employs a polymer dispersion liquid crystal.